Field of the Invention
The present invention relates generally to a process for coating conductive workpieces and for an apparatus using this process.
Description of the Related Art
Processes are known for coating workpieces in which the coating material is deposited from a vapor onto the workpiece. To reduce the strain on the workpiece caused by high temperatures occuring during the coating process, the coating material is evaporated in a vacuum within a vessel by different physical methods. These are the so called PVD (physical vapor deposition) processes. Different methods for creating the vapor from a target or coating supply are: sputtering (such as by ion bombardment), arcing, electron beams, etc. By applying a high voltage, a plasma can be created containing ions of the vapor. These ions are handled by electric and magnetic fields to get special coatings on the workpieces. Using such electrical and magnetic fields can increase the deposition rate.
Coating processes using a plasma create an undesirable high temperature in the workpiece and in the coating, since the kinetic energy of the ions in the plasma is absorbed at the respective surfaces. The high kinetic energy also results in removal of at least some of the deposited coating. Normally, a high deposition rate at a low temperature for the workpiece is desired so that the above-mentioned properties are supressed.
In the known devices, a plasma is created by electric discharge. The electric discharges are ignited between two electrodes; the evaporated material in the vessel between the electrodes being ionized so that an electric current passes through the plasma. The electric current supports the plasma; but on the other hand, the energy input into the plasma by the current leads to high temperatures both in the electrodes and within the vessel. Therefore, the chamber walls of the vessel, as well as the workpieces, generally have to be cooled. Pieces to be treated which have a high surface-to-volume ratio can easily become overheated by the ion bombardment of the plasma. The consequence of such over heating is a high rate of damaged parts in a batch of workpieces. Alternately, it becomes necessary to put only similar parts into each load. A further problem in the known processes is that a high temperature gradiant occurs between the work load or batch and the cooled chamber walls, which results in deposition conditions which are very different for each part. Therefore, the coating is unequal on different workpieces within the vessel.